Diabetes mellitus (diabetes) is a leading cause of morbidity and mortality in the adult population. This is primarily because diabetic patients tend to develop vascular complications that involve the kidneys (diabetic nephropathy), the retina (diabetic retinopathy), as well as large and small blood vessels in other organs (macro- and micro-vascular disease) including nerves (diabetic neuropathy). It is well established that the vascular complications of diabetes are caused by elevated blood glucose levels over long periods of time. Elevated blood glucose levels affect proteins by a nonenzymatic process known as glycation. Glycation, the non-enzymatic attachment of glucose to proteins, is considered a major post-translational modification causing tissue damage in diabetic subjects. Glycation involves the reaction of glucose and/or other reducing sugars with amino groups in proteins resulting in the formation of a Schiff's base or aldimine. This labile Schiff's base can cyclize to a more stable glycosylamine adduct. The function of the glycated protein may be impaired, depending on the location of the amino groups affected. Glycation of key regulatory proteins, such as those which prevent activation of the complement system, is believed to contribute to the manifestations of diabetes mellitus. Thus, compositions and methods which help measure the extent of protein glycation of key regulatory proteins of the complement system are considered valuable clinical tools to assess glycemic control over periods of different length and the efficacy of diabetes treatment.